web.xml

Other

The Loader Component

Table of Contents

Introduction

The Loader element represents the web
application class loader that will be used to load Java
classes and resources for your web application. Such
a class loader must follow the requirements of the Servlet
Specification, and load classes from the following locations:

From resources made available by Catalina to all web
applications globally.

A Loader element MAY be nested inside a Context
component. If it is not included, a default Loader configuration will be
created automatically, which is sufficient for most requirements.

For a more in-depth description of the class loader hierarchy
that is implemented by Catalina, see the ClassLoader HowTo.

The description below uses the variable name $CATALINA_BASE to refer the
base directory against which most relative paths are resolved. If you have
not configured Tomcat for multiple instances by setting a CATALINA_BASE
directory, then $CATALINA_BASE will be set to the value of $CATALINA_HOME,
the directory into which you have installed Tomcat.

Attributes

Common Attributes

All implementations of Loader
support the following attributes:

Attribute

Description

className

Java class name of the implementation to use. This class must
implement the org.apache.catalina.Loader interface.
If not specified, the standard value (defined below) will be used.

delegate

Set to true if you want the class loader to follow
the standard Java2 delegation model, and attempt to load classes from
parent class loaders before looking inside the web
application. Set to false (the default) to have the
class loader look inside the web application first, before asking
parent class loaders to find requested classes or resources.

reloadable

Set to true if you want Catalina to monitor classes in
/WEB-INF/classes/ and /WEB-INF/lib for
changes, and automatically reload the web application if a change
is detected. This feature is very useful during application
development, but it requires significant runtime overhead and is
not recommended for use on deployed production applications. You
can use the Manager web
application, however, to trigger reloads of deployed applications
on demand.

NOTE - The value for this property will be
inherited from the reloadable attribute you set on
the surrounding Context component,
and any value you explicitly set here will be replaced.

Standard Implementation

The standard implementation of Loader is
org.apache.catalina.loader.WebappLoader.
It supports the following additional attributes (in addition to the
common attributes listed above):

Attribute

Description

loaderClass

Java class name of the java.lang.ClassLoader
implementation class to use. Custom implementations must extend
org.apache.catalina.loader.WebappClassLoaderBase.

If not specified, the default value is
org.apache.catalina.loader.WebappClassLoader. The
default loaderClass is not parallel capable, which
means that loading a class from this classloader is performed by one
thread at a time. A parallel capable loaderClass is
available and can be used by specifying
org.apache.catalina.loader.ParallelWebappClassLoader.

searchExternalFirst

Set to true if you want repositories outside
of WEB-INF/classes and WEB-INF/lib to
be searched first. Default value is false.